No plastic pipe, and all TRV's except one in the hallway, and there is a bypass with a gate valve on it. The plumber has looked at this, and recommended that I upgrade from a 3 way to 2 x 3 way valves with an automatic bypass. But it used to be fine! The rad valves are 15 years old, I am guessing. However, since the pump is a few centimeters upstream of the cold feed I am assuming this is the only area that could possibly 'suck' in air? I must admit, it is one particular TRV upstairs that makes a lot of noise, and the same rad that gets most gas - but no negative pressure surely? - the F&E tank is in the loft. PS the water is clear with black flecks and occasional translucent 'floaty' bits!